1. Field of the Invention
The present invention relates to a method and apparatus for detecting motion in an image display device, and more particularly, to a motion detection method and apparatus for respectively determining luminance motion factors and chrominance motion factors to further determine motion factors of images.
2. Description of the Prior Art
With technological advances in display technology, video processing, and integrated circuit fabrication, in tandem with the rapid development of wireless networking, users can view their favorite movies and television programs on a video display device (such as a television) any time, any place. Thus, information and entertainment become increasingly accessible, and user requirements for picture quality increase in like manner.
The human eye has four different types of light receptor, of which three are used for distinguishing light of different wavelength (the fourth is only used under dim lighting conditions, and cannot discern colors). In other words, all light visible to the human eye can be fully described by three axes. Thus, when displaying a picture, only red, blue, and green (RGB) light information output is needed, when speaking in terms of the human eye, to show an image of realistic quality. However, to reduce bandwidth and ensure compatibility, the prior art color television broadcast system does not directly output RGB signals, but instead outputs a composite signal. The “composite” signal is an output signal that is a mix of a luminance signal and a chrominance signal, which is compatible with black-and-white and color television systems, and also conserves bandwidth.
The earliest television was the black-and-white television. Later, when color television systems were being developed, to promote compatibility between black-and-white television signals and color television signals, black-and-white (luminance) signals and color (chrominance) signals were separated. In this way, a black-and-white television needed only to decode the incoming luminance signal from a television station in order to display a picture. Color televisions would decode both the luminance signal and the chrominance signal together in order to display a color picture. Because the human eye is more sensitive to luminance than chrominance, or in other words, the human eye requires less color resolution than black-and-white resolution, the color signal does not require as much bandwidth as the black-and-white signal. Thus, by taking advantage of the human eye's relative insensitivity to color, transmission bandwidth can be reduced and adapted in black-and-white and color televisions.
Taking the National Television Standards Committee (NTSC) standard as an example, NTSC originally used a YIQ color space. The YIQ color space uses quadrature modulation to synthesize a common spectrum intermodulation signal I with a quadrature signal Q to form a single chrominance signal C. The chrominance signal C is then added to a luminance signal Y, and with an accompanying horizontal synchronization pulse, a blanking pulse, and a color burst, the composite signal is generated. The NTSC standard adopts a 6 MHz channel bandwidth, with 4.2 MHz reserved for the luminance signal Y, 1.6 MHz given to the intermodulation signal I, and 0.6 MHz appropriated to the quadrature signal Q. In contrast to the NTSC standard signal, the Phase Alternating Line (PAL) standard adopts a YUV color space. To increase picture quality, a color phase of the chrominance signal is alternately set as positive and negative for each successive scanline. The PAL standard uses an 8 MHz channel, allocating 5.5 MHz to the luminance signal Y and 1.8 MHz to a signal U and a signal V.
Thus, by splitting the luminance signal and the chrominance signal, then transmitting the signals together, the transmission bandwidth can be reduced, and the transmitted signal can be used in both black-and-white and color televisions. Correspondingly, a receiving end need only comprise a circuit such as a comb filter, for isolating the luminance signal Y and the chrominance signal C, in order to play both black-and-white and color television. However, the composite Y/C signal has one large problem, which primarily lies in the fact that high-frequency components of the luminance signal Y overlap with the frequency spectrum of the chrominance signal C. This makes it difficult for the receiving end to accurately and completely separate the luminance signal Y and the chrominance signal C in their original forms from the composite signal Y/C. Ultimately, this inability to separate the luminance signal Y from the chrominance signal C results in flaws in the picture. For example, if the luminance signal Y is processed as part of the chrominance signal C, a cross-color artifact is produced, and the picture will exhibit a rainbow effect. Likewise, if the chrominance signal C is processed as part of the luminance signal Y, a cross-luminance artifact is produced, resulting in a horizontal or vertical dotted line in the static picture, and a meshed image in the motion picture.
As well known in the art, in the NTSC image system, phases of sub-carriers of the chrominance signal convert 180 degree between adjacent image frames. In this way, when the luminance information is mistakenly decoded as the chrominance information (cross-color artifact), the chrominance information oscillates with two complementary colors such as red and green. That is, due to 180-degree phase difference, the luminance information becomes two complementary colors in chrominance spectrum. Similarly, when the chrominance information is mistakenly decodes as the luminance information (cross-luminance artifact), the effect of 180-degree phase difference can also be observed in the luminance information of adjacent frames. Since the phase difference is 180 degree, the cross color artifact can be suppressed by subtracting the composite video information of adjacent image frames for returning the original chrominance information. Likewise, the cross color artifact can be suppressed by averaging the composite video information of two adjacent image frames. However, this technique works only when the image is static or still. Therefore, in order to effectively enhance the quality of static images and motion images, the prior art luminance and chrominance separation circuit selects an appropriate filter for performing filtering operations according as the image is static or in motion.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art three-dimensional luminance and chrominance separation circuit 10 for the NTSC image system. The three-dimensional luminance and chrominance separation circuit 10 includes a low-pass filter 100, a two-dimensional comb filter 102, a three-dimensional comb filter 104, an edge detector 106, and a filter selection unit 108. The edge detector 106 can determine whether corresponding pixels are corresponding to horizontal or vertical “image edges” (i.e. boundaries of large gray value differences), and decide weighting values of signals Y1 and C1 outputted by the low-pass filter 100 and signals Y2 and C2 outputted by the two-dimensional comb filter 102, so as to output luminance signals Y4 and chrominance signals C4. The luminance signals Y4 and the chrominance signals C4 outputted by the edge detector 106 can improve the image quality of motion pictures, while the three-dimensional comb filter 104 can enhance the image quality of static pictures. The filter selection unit 108 includes a motion detector 110 for determining whether the image is in motion or static. The filter selection unit 108 can determine weighting values of the signals Y4 and C4 outputted by the edge detector 106 and signals Y3 and C3 outputted by the three-dimensional comb filter 104, so as to output luminance signals Y and chrominance signals C.
In the prior art, the methods for the motion detector 110 to determine whether the image is in motion or static can be generalized into two kinds. First, since phases of chrominance sub-carriers convert 180 degree between adjacent frames in the NTSC image system, the motion detector 110 can determine whether the image is in motion or static according to differences of the composite video signals between two frames separated by one frame. However, with this method, some motion images may be mistakenly decoded as static images. For example, an object jumps or swings between two positions (such as a swinging squirrel tail), so that images of two frames separated by one frame are the same or similar. In this case, the motion detector 110 will determine motion images as static images incorrectly.
Second, since the frequency spectrum of the chrominance signals lies in a high-frequency part of the composite video signals, the motion detector 110 can determine whether the image is in motion or static according to low-frequency components of the composite video signals (namely, low-frequency luminance signals) between adjacent frames. In this case, if a motion image has chrominance differences but no luminance differences, the motion detector 110 will determine motion images as static images incorrectly.
Therefore, when motion detection results of the motion detector 110 mistakes, the filter selection unit 108 introduces an incorrect weighting ratio for the signals Y4 and C4 and the signals Y3 and C3, and further influences the luminance signals Y and the chrominance signals C, so as to deteriorate the image quality.